The present invention relates to a scoring device for a gypsum board production machine, and more specifically, to such a device which continuously forms a score on a sheet of paper for gypsum board liner traveling under tension.
Gypsum boards, each having a gypsum core covered with sheets of paper for gypsum board liner, are known as building materials for interior finish. The gypsum boards are mass-produced by gypsum board production machines, and circulated on the market. In general, the gypsum board production machine is provided with a conveying device for continuously conveying the sheet of paper for gypsum board liner (lower sheet); a scoring device for scoring the sheet on its side edge portions; a mixer for preparing a gypsum slurry; a folding device for folding the sheet to configure edge portions of the board; a laminating and forming device for laminating another sheet of paper for gypsum board liner (upper sheet) onto the gypsum slurry; a cutting device for cutting a continuous web-like form into green boards having a predetermined board length; a drying device for drying excessive water contained in the boards; and a delivery device for delivering products, which are the boards finely cut to be a predetermined product size.
The scoring device continuously forms scores on the sheet of paper for gypsum board liner, which travels under tension. The scoring device is known, which is constituted from a folding-line forming part such as a disc-shaped whetstone or a pressing part, and a driving device for rotating or energizing the forming part. The forming part forms a notch or crease on the sheet of paper (lower sheet) to provide the score thereon. The scoring device also has manually operated lifting means for displacing the forming part in a direction of the thickness of the sheet, and manually operated moving means for displacing the forming part in a widthwise direction of the sheet. The position of the score to be formed on the sheet is adjusted by manual operation of the lifting means and the moving means.
As the scoring device of the gypsum board production machine, a folding-line forming device is disclosed in Japanese Patent Laid-open Publication No. 58-86937 (JP-A-58-86937), in which the sheet of paper for gypsum board liner is passed between a folding-line forming part and a backup roller by a conveyance device so that a folding line is formed on the sheet under pressure of the forming part.
Further, a method of bending a thick sheet such as a corrugated fiberboard for producing a box or the like is disclosed in Japanese Patent Laid-open Publication No. 08-150675 (JP-A-08-150675), in which a notch in a form of wedge is made on the sheet by a knife and the sheet is bent accurately on the notch.
The sheet of paper for gypsum board liner is a sheet of about 0.3 mm in thickness and about 200 g/m2 in weight. The knife as disclosed in JP-A-08-150675 is merely a knife used for forming the wedge-shaped notch on the thick sheet of paper such as a corrugated fiberboard, and therefore, such a knife cannot be used for scoring the sheet of paper for gypsum board liner.
On the other hand, the device disclosed in JP-A-58-86937 is a creaser type of scoring device which presses the folding-line forming part against the sheet of paper for gypsum board liner so that a folding line is formed by locally pressing action on the sheet. This type of scoring device is finely adjusted by a skillful operator, and the adjustment depends on experience of the operator.
In recent years, reduction of a thickness of the sheet of paper has been desired for reduction of production costs, improvement of recycling recovery of gypsum boards, and so forth. Under such circumstances, a thin sheet of paper of about 0.2 mm in thickness and about 140 g/m2 in weight may be used as the sheets of paper for gypsum board liner. In addition, the tension imposed on the sheet has tended to be increased in relation with speeding-up of the gypsum board production line in recent years. In such a production condition, load on the sheet of paper is increased and a fracture of the sheet is apt to be caused. Therefore, it is difficult to employ the creaser type of scoring device in which the fine adjustment depends on the experience of the skillful operator.
Further, in consideration of workability of gypsum boards, it is preferable that the edge portion of the gypsum board has a precise and sharp corner. In general, the angle of the corner of the edge portion is normally managed to be a substantially right angle. However, in a case of the creaser type of scoring device, the angle of the edge portion is apt to exceed 90 degrees, whereby the gypsum boards having an undesirable appearance may be produced as the result of corners without sharpness. This results in decreased yields and low productivity.
In consideration of such problems, whetstone types of scoring devices are usually employed in Japan.
FIGS. 10 (A) and 10 (B) are cross-sectional views schematically showing a structure of a whetstone type of scoring device. FIG. 10 (C) is a partial plan view of a backing block. FIGS. 10 (D) and 10 (E) are partial cross-sectional views of a sheet of paper for gypsum board liner showing cross-sections of a score on the sheet.
The scoring device has a disc-shaped rotary blade A concentrically fixed on a rotary drive shaft B. A backing block C is positioned under the blade A. The sheet of paper for gypsum board liner D travels on the block C in a direction of an arrow E. The blade A rotates in a normal rotational direction (a direction of an arrow) or a reverse direction with rotation of the shaft B. The blade A grinds an upper layer of the sheet D to form a groove G on the sheet D.
This type of scoring device allows a relatively sharp corner to be formed on the edge portion of the gypsum board. Further, this type of scoring device facilitates management to make a substantially right angle in the edge portion. Therefore, such a scoring device is preferably employed in order to prevent a yield of production from being decreased due to the inaccurate angle of the edge portion.
However, the blade A and the block C in this type of scoring device are apt to be worn. Therefore, the blade and the block have to be replaced with new parts relatively frequently, in relation with the tendency of speeding-up of the gypsum board production line.
Further, in the whetstone type of scoring device, the depth of the groove G is apt to change, owing to a slight eccentricity of the shaft B and the blade A, change in the tension on the sheet D and so forth. Therefore, the groove G thus formed tends to have an irregular depth as shown by a dotted line on FIG. 10 (E). This tendency has been noticeably observed especially in recent years, owing to reduction in the thickness of the sheet of paper for gypsum board liner and speeding-up of the gypsum board production line. This results in a problem of discontinuous or intermittent folding lines formed on the sheet.